Hi , I'm James DVM MRCVS and would like to help you with your question
Immunovet is an adaptation from human medicine used mainly as supplementation for cancer patients.
It has been known to help with patients suffering as well from Autoimmune disease which by the sound of it - Fidget - is suffering as well.
As it is currently regarded as a supplement it can not come instead of medication intended against Fidget condition but it can sure be added.
Histiocytosis is hard to classify as it falls between being an autoimmune disease to an idiopathic granulomatous which can stop and start when ever it wishes too .
So in answer to your original question - You can sure give it a try to see if immunovet helps relieve the symptoms of Fidget's disease, you have nothing to lose and a lot to gain :)
I hope this helps
In case you have any further questions , please let me know and I shall do my best to be back ASAP.
I wish you both all the best .
p.s - before leaving our conversion , please do not forget to rate it